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Evan Andersen 5th October 2001 12:03 AM

I'm building a PA subwoofer using an Eminence Sigma Pro-18" (see other thread), and I want to protect it with a fuse. The driver handles 500w/RMS, and the DC resistance is 6.4 ohms. There is no crossover because I am bi-amping. How can I figure out what fuse rating to use, and should it be slo-blow or fast-blow?


AndrewJ 8th October 2001 09:00 PM

Evan, assuming the driver will truly handle 500w RMS long term, then the fuse rating can be calculated as follows:
The current drawn at 500W into 6.4ohms is I=sq root (W/6.4)
This works out at 8.8 amps, so to be on the safe side try a 7.5 amp slow blow fuse.

JBL 9th October 2001 01:26 AM

Remember that a fuse on a speaker will always blow after a time. It can get very annoying(trust me) when it's come to change it and you don't have some.

Evan Andersen 9th October 2001 04:32 PM

Hmm..I was not aware of this problem. Maybe I'll nix the idea, or keep an emergency wad of aluminum foil on hand! How long will a slow-blow fuse usually last in this application?


JBL 9th October 2001 11:20 PM

hehe aluminium foil. I have alot of experience with aluminium foil. It's actually depend. I ahve see case when they just didn't last a week. It depend if you push your system hard. Also playing with aluminium foil is not very reccomended. You will always have a problem when it's they foil in place.

You can also use a higher value fuse but passed a point it's going to be the speaker protecting the fuse(tweeter make excellent fast acting fuse btw :).

swede 10th October 2001 11:02 AM

Why not use a fast auto-fuse (the kind with a small button to press when you've removed the load on the speaker)? They usually last more than 1000 times, before you need to replace them.


hifi 10th October 2001 11:16 AM

but those are usualy rataed att above 100amps...?

swede 10th October 2001 11:56 AM

Not the ones I mean.

They look like:

<img src=>

And they are rated from 2 A and up.


paulb 10th October 2001 01:26 PM

That's a circuit breaker. It may affect the sound.

swede 10th October 2001 05:07 PM

Ok, never mind. ;=)

I thought they'd do.
But I've seen similar fuses on older Yamaha speakers.


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